The specific aim of this research program is to identify chemical alterations produced by ionizing radiation in the sugar moiety of mononucleotides and assess the role of the base and phosphate in the production of these alterations. Ultimately this information will contribute to achieving the long-term objective of characterizing chemical events which result in single strand breaks. The approach to this problem is to identify, by ESR and ENDOR, free radical products and reactions in single crystals of mononucleotides and other DNA model compounds. One of the specific objectives is to determine a comprehensive reaction scheme accounting for all major free radical products in both 3' cytidylic acid and 10K and 300K. Also under study is aminoethanol-phosphate in the form of single crystals. This compound serves as a model for studying the involvement of the phosphate in free radical damage at the phosphoester bond of nucleotides. The ESR spectra of single crystals of 5'dCMP, x-irradiated at 11K and observed at 6K, exhibit signals from three distinct free radicals. Analysis of ENDOR spectra from two of these radicals indicates that these result from oxidation and reduction of the cytosine base. The third free radical has ESR spectra characteristic of an oxygen centered defect and is thought to be a hole located primarily on the C3'-03' oxygen, but shared with a neighboring HO-PO3-CH 2-group.